1. Field of the Invention
The field of art to which this invention pertains includes pressure regulator or inching valve means for use in fluid operated control systems. More particularly, the improved inching valve of this invention is constructed so that it positively restricts the operation thereof to pressures below the maximum desired clutch slip pressure.
2. Description of the Prior Art
Briefly, this invention constitutes an improvement in pressure regulating or inching valves, with such valves often being used in the prior art for regulating pressure to a fluid operated clutch of the type often found in the constant-mesh powershift transmissions of industrial vehicles, such as lift trucks and the like. Specifically, such prior art control valves are frequently constructed to permit an immediate relief of a portion of the clutch application pressure followed by a regulated decrease or increase in the fluid pressure applied to the fluid clutch whereby the latter may be gradually disengaged or engaged by the operator. One particular use of this type of regulating valve contemplates the control of a fluid-pressure actuated clutch for the transmission of, for example, a lift truck where it is often necessary to hold the truck stationary while the lift fork is operated at a speed corresponding to full engine speed without shifting the transmission to the neutral position. It is desirable, in such circumstances, to be able to precisely move or "inch" the vehicle in either the forward or reverse direction in order to achieve accurate positioning in the handling of materials.
Various controls and regulating valves for performing the above-noted function in conjunction with powershift transmissions applying fluid pressure actuated transmission clutches have been disclosed previously and are exemplified by U.S. Pat. No. 4,143,127 to Frost, as well as U.S. Pat. No. 3,990,549 to Sisson et al, and U.S. Pat. No. 4,006,805 to Zeller et al. In order to carry out their regulating valve function, the valve means, such as described in these patents, have been provided to affect improved gradual engagement or disengagement of the vehicle clutch and the relief of excess pressure in the system by operating the valve means in conjunction with the application and release of the vehicle brakes. In such structures, where the actuation of the valve means may be either hydraulic or mechanical, when the brakes are actuated the valve means shifts to a position partially interrupting fluid flow to the clutch and thereby effects controlled partial deactuation of the clutch by maintaining a relatively low fluid pressure, thus providing control of the torque to the wheels of the vehicle.
In these prior art patents, there is an initial direct mechanical connection between the inching valve spool and its brake-controlled actuating means so that the operator can, by judicious brake pedal actuation, control that portion of the inching valve pressure drop between pump pressure and the desired maximum slip pressure to the detriment of clutch life.